Zheng Mingda, Yan Yuebo, Li Ya, Yao Lihui, Duan Jinguan*
(Ningbo Engineering College, Materials Department, Ningbo, 315016)
Summary:This article describes a "two-step" process for preparing a resin for bonding metal-plastic composite pipes, utilizing reactive extrusion and physical mixing techniques. The prepared resin was characterized using techniques such as infrared spectroscopy, contact angle measurement, and scanning electron microscopy, and also subjected to application experiments. The results demonstrate that the prepared resin exhibits excellent bonding properties and fully meets the requirements for market applications.
Keywords:- Adhesives; metal-plastic composite pipes; grafting reaction.
Introduction
"Hot melt adhesive refers to a type of adhesive that is solid at room temperature but becomes liquid upon heating. After application, wetting, pressing, and cooling, it can achieve bonding in a short period.[1]。Hot melt adhesives are categorized based on their applications, primarily including:* Polyolefin (PO) type hot melt adhesives* Ethylene/Vinyl Acetate (EVA) type hot melt adhesives* Polyamide type hot melt adhesives* Butyl type hot melt adhesives[2-5]"Polyolefin hot melt adhesives are primarily used for bonding metal-plastic composite materials. With the development of society and technology, metal-plastic composite materials have evolved from steel-plastic composite coatings and aluminum-plastic composite materials (such as aluminum-plastic composite pipes, bags, and sheets) to steel-plastic composite and copper-plastic composite materials. The market demands for metal-plastic composite materials are increasing, particularly the performance requirements for the adhesive resin used in metal-plastic composite bonding, which include bonding performance, heat resistance, and environmental performance (low odor, low volatility, and low toxicity). Due to the presence of polar groups such as -OAl, -OH, and -COOH on the surface of metal materials, and the non-polar nature of polyolefins, bonding between the two is generally difficult.[6-8]。Therefore, it is common to graftMaleic anhydride onto polyenes to introduce polar groups, and then use the modified polyenes as a base for adhesives, which can form a long-lasting bond with metals. Currently, domestic adhesives of this type have relatively low bonding strength and poor aging resistance, especially for metals such as steel and copper. This is because the grafting modification method alone cannot achieve high bonding performance for adhesive resins. The bonding performance of grafted polyene adhesives is related to the grafting rate. The higher the grafting rate, the stronger the bonding performance. When the grafting rate exceeds a certain value, the bonding performance of the adhesive resin tends to stabilize with the increase in grafting rate.
Additionally, achieving high grafting rates is challenging during the melt grafting reaction due to the presence of crosslinking side reactions. Increasing grafting rates typically leads to increased gel content, which significantly affects the flowability of the adhesive resin. Furthermore, melt grafting of maleic anhydride onto polymers typically involves a high residual amount of maleic anhydride in the grafted polymer (with an effective grafting efficiency of approximately 30%). This results in significant odor and environmental impact due to the volatilization of maleic anhydride during both the grafting process and the use of the adhesive resin. The presence of significant monomer residue also affects the adhesion performance of the adhesive resin.[9,10]。Therefore, the development and promotion of low-odor, low-volatile bonding resins are of paramount importance.
Metal-plastic composite materials, especially metal-plastic composite pipe systems, are increasingly used in the transportation of heat media and pressure media. Therefore, the stringent application conditions for metal-plastic composite pipe systems require higher performance, particularly in terms of the bonding layer's thermal resistance. This necessitates the selection of bonding resins with high melting points. However, resins with high melting points often exhibit poor branching and bonding properties. Resolving these trade-offs is a key focus for researchers and manufacturers in the bonding resin field. Therefore, developing and producing high-performance, high-temperature-resistant, and environmentally friendly metal-plastic composite bonding resins with excellent bonding properties holds significant market potential.
based on the above issues, this article adopts a "two-step" process, utilizing reaction extrusion technology to produce a bonding resin for metal-plastic composite pipes. The resin was then characterized and analyzed at both the micro and macro levels, and application experiments were conducted. The results demonstrate that this technology can produce a superior bonding resin.
2. Experimental Part
2.1 Main Experimental Materials
The main raw materials and their sources used in this document are listed in Table 1.
Table 1: Main Experimental Raw Materials
2.2 Experimental Equipment
The main experimental instruments used in this article, along with their model numbers and manufacturers, are listed in Table 2.
Table 2: Main experimental instruments
2.3 Characterization Methods
Fourier Transform Infrared Spectroscopy: Utilizing the reflectance testing method of Fourier Transform Infrared light to analyze the structure and composition of the grafted material obtained through melt-bonding and semi-solid bonding techniques, and to characterize the grafting rate.
Melt Flow Index Determination: The melt flow index is determined using a melt flow index tester to characterize the processing and flowability of branched materials produced by different branching methods. The test is performed according to GB/T 3682.
Contact Angle Measurement: The contact angle between the contact angle measurement instrument and the test material is measured to characterize the hydrophilicity of the brazed material produced by the melt-brazing method.
Scanning Electron Microscopy (SEM) Testing: The SEM was used to observe the microstructure of the base resin (PE) and modified adhesive resin after bonding to the copper sheet, focusing on the microstructure of the bonded surface.
Adhesion surface spectroscopy measurement: The spectrometer analyzes the elemental composition of the adhesion samples (after etching with dimethylbenzene and etching with concentrated sulfuric acid, removing the hot melt adhesive and copper, respectively) to characterize the adhesion properties of the modified adhesive resin.
Peeling Strength Measurement: The peeling strength is measured using a peeling strength testing machine, following the standard GB/T 7122. Samples in the form of "copper foil-adhesive resin-copper foil" are used to investigate and test the peeling strength of modified adhesive resins, thereby characterizing their adhesive properties.
Mechanical performance testing of hot melt adhesive: Testing the tensile strength and elongation at break of modified adhesive resins to characterize their mechanical properties.
2.4 Preparation Method
The adhesive resin is prepared in a two-step process: the first step involves preparing the grafting material, and the second step involves preparing the adhesive resin. The preparation process of the grafting material:
The process of preparing adhesive resin:
3. Results and Discussion
3.1 Infrared Characterization of Grafted Polyethylene
Figure 1 shows the infrared spectra of two grafting agents using the semi-solid grafting method, and Figure 1 shows the infrared spectra of two grafting agents using the fusion grafting method. From Figure 1, it can be clearly seen that MAH is obviously grafted onto the PE. The experimental results obtained using different initiators are essentially consistent.
Figure 1: Infrared spectra of polyethylene before and after grafting.
3.2 Characterization of Grafted Polyethylene and the Polarity of Adhesive Resin
To further demonstrate whether the PE base material has undergone grafting, this paper employed a contact angle meter to test the obtained grafted materials and adhesive resins. Figure 2 shows images of the grafting method, adhesive resin, and pure resin contact angles. From Figure 2, it can be seen that the contact angle of the grafted PE is 48.90°. The contact angle of the grafted PE is significantly lower than the contact angle of the pure PE (77.7°), indicating that the grafted PE has polarity. This is due to the grafting of polar MAH groups onto the PE molecules. The final contact angle of the obtained adhesive resin is 56.2°, which is significantly lower than the contact angle of the pure PE, indicating that the obtained adhesive resin has high polarity. Higher polarity can improve the adhesion strength of the adhesive resin.
Figure 2: Contact angle of the adhesive resin before and after grafting with polyethylene.
3.3 Adhesion Performance Testing of Adhesion Resin
To further investigate the bonding characteristics and strength of the adhesive resin, Figure 3 shows images of the "sandwich" bonding sample surfaces after detachment. As shown in Figure 3, the adhesive resin is firmly bonded to the copper plate, demonstrating strong bonding strength.
Figure 3: Disassembly sample of "sandwich" consisting of copper foil - adhesive resin - copper foil.
Figure 4 shows concentrated H2SO4The EDS spectrum of the adhesion resin in the peeling experiment shown in Figure 3 (where the copper plate was etched away) reveals that concentrated sulfuric acid can corrode the copper plate, leaving behind a PE plastic sample. Analysis of the EDS spectrum on the surface of the PE sample indicates that it primarily contains C and copper elements. This suggests that the MHA molecules on the PE surface have formed chemical bonds with the -OH groups on the copper plate, resulting in a chemical reaction. After the unreacted copper is corroded away, the copper elements involved in the chemical reaction remain. This demonstrates that the PE adhesion resin indeed reacted chemically with the copper plate, thereby improving the adhesion strength.
Figure 4, concentrated H2SO4Spectrum of the bonded surface after etching (etching removed the copper plate)
Figure 5 shows the X-ray photoelectron spectroscopy (XPS) results of the bonded joint surface after hot toluene etching (where the adhesive resin was removed). As is well known, hot toluene can dissolve PE resins. After hot toluene etches the "sandwich" adhesive sample, the free PE molecules are removed by the toluene, and the resulting sample's XPS spectrum shows that the copper sheet primarily contains copper and C elements. This indicates that the PE adhesive resin has reacted chemically with the copper.
based on Figures 4 and 5, the following conclusions can be drawn: The adhesive resin reacted chemically with the copper material, thereby increasing the adhesion strength.
Figure 5: Energy dispersive X-ray spectroscopy (EDS) of the bonding joint surface after etching with hot dimethylbenzene (etching removed the bonding resin)
Figure 6 shows SEM photos of the remaining copper sheets after peeling, for both pure PE and different adhesives. As shown in Figure 3-13, after peeling pure PE, there is almost no residue on the copper sheet (a), while after peeling the adhesive, there is a significant amount of residue on the copper sheet, which clearly indicates that the adhesive and copper have a very strong bonding effect. This suggests that the adhesive and copper metal have a high bonding strength and detachment strength, as predicted in this study.
Figure 6: SEM images of the adhesive resin: (a) pure PE (b) adhesive resin
b
3.4 Comprehensive Performance Characterization of Adhesion Resins
Adhesive resins have high cohesion, which can improve the bonding strength and peel strength between bonded materials. Therefore, studying the mechanical properties of adhesive resins is of great importance for the performance of adhesive resins. Table 3 and Figure 7 show the mechanical properties of the adhesive resins. From these data, it can be seen that the adhesive resins studied have high mechanical properties, which play an important role in promoting bonding performance.
Table 3: Mechanical Properties of Adhesion Resins
Figure 7: Load-displacement curve of the adhesive resin
3.5 Application test of adhesive resin
Send samples of the adhesive resin to the collaborating manufacturer for testing. After wire drawing and forming, based on experience, the bonding strength is assessed to meet the requirements, and approval is granted for direct extrusion of Φ110 fittings. During the extrusion process, the wire, which is already coated with resin (possibly a bonding agent like "Bonding Agent"), is used as a reinforcing mesh (see Figure 8). The adhesive resin can be applied using the existing process.
Figure 8: Processing image of adhesive resin
Wire Corrosion Test: After plasticization for 3-7 days, remove the adhesive layer and inspect the internal wires for corrosion. The adhesive resin should meet the required bonding strength.
Pipe Material Testing:
Tear test
- Take samples from different locations of the fittings.- After peeling off the ends, perform a tearing test.- Perform the experiment twice: once after squeezing the pipe for 2 days and once after 30 days.- Tensile speed: 100 mm/min.- The results should be > 20 N/mm.
Explosive testing
Explosion-proof fittings, requiring an explosion pressure greater than 4.8 MPa (3 nominal pressures).
Ambient static pressure
At room temperature, applying a water pressure of 3.2 MPa (2 inch water column pressure) for at least 1 hour, the fittings will not rupture.
Water penetration test
Remove the outer layer of the pipe material, exposing the adhesive layer, and apply a pressure of 2.4 MPa (1.5 bar) at this location. After 165 hours, no liquid has penetrated the interface between the adhesive and the steel wire.
High-Temperature Static Pressure Test
Secure both ends of the pipe, immerse them in 80°C hot water, and maintain a continuous pressure of 1.92 MPa (1.2 bar) within the pipe for 165 hours. The pipe should not leak.
99% of the test results within the specified pressure range showed that the bonding resin maintained its adhesion for 141 hours, while commercially available bonding resins maintained adhesion for 143 hours. When calculated based on the total time with the fittings installed, both the bonding resin and the commercially available bonding resin exceeded 165 hours. The two fittings using commercially available bonding resin detached and leaked due to the clamps, resulting in the termination of the test. The surfaces did not show any damage. One fitting had a noticeable protrusion, while the other appeared largely unchanged. The bonding resin samples prepared in this experiment exhibited protrusions and damage, including fraying, but the fittings were installed for over 165 hours. The experimental results are shown in Figure 9. based on the above application test report, the bonding resin studied and prepared in this article demonstrates excellent application results and fully meets the requirements for the use of PE-copper composite pipes.
Figure 9: Bond strength test of adhesives: (a) commercially available adhesive (b) adhesive used in this experiment
4、Conclusion
By employing different grafting techniques, various adhesives were prepared. Through the performance evaluation and application experiments of the adhesives, the following conclusions can be drawn: 1) Using melt grafting can produce PE grafting materials with high grafting rates; 2) Infrared spectroscopy, contact angle, SEM, and energy spectroscopy tests indicate that the obtained grafting resins and adhesives have high bonding strength and detachment strength. 3) Due to the suitable melt index, melting point, and mechanical properties of the adhesives studied, the obtained adhesives have high wetting and cohesion. 4) Through processing and application experiments, the adhesives studied and prepared in this paper have excellent application results and fully meet the requirements for the use of PE-copper composite pipes.
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Study of Adhesive Resin Applied in Metal-Plastic Pipes
Zheng Mingda, Yan Yuebo, Li Ya, Yao Lihui, Duan Jingkuan
(Institute of Materials Engineering, Ningbo University of Technology, Ningbo 315016)
ABSTRACT
This article describes the preparation of an adhesive resin for metal-plastic pipes using a "two-step" method involving reactive extrusion and physical blending technologies. The Fourier Transform Infrared Spectroscopy (FTIR), Contact Angle (CA), and Scanning Electron Microscope (SEM) were used to characterize the adhesive resin, and application experiments were conducted in a factory setting. The results demonstrate that the adhesive resin exhibits excellent properties and is suitable for market needs.
Key words- Adhesive resin- Metal-plastic pipe- Grafting reaction